In a pipeline flowing a product, such as steam or natural gas, it often becomes necessary to measure the flow of the product through the pipeline. One widely employed technique for measuring the flow through the pipeline involves the use of an orifice plate or restriction in the pipeline to create a pressure drop. The upstream or downstream pressure is sensed to provide a reference signal, called the static pressure signal, to the flow monitoring instrument. The pressure differential across the orifice is measured and compared to the reference signal to calculate the volume flowing through the pipeline.
It is well known that a unitized manifold body which incorporates a plurality of valve assemblies between a network of internal flow passages is desirable to minimize leak points and simplify installation. A typical double block instrument manifold is described in U.S. Pat. No. 4,453,417 to Moyers et al.
Depending upon the fluid being measured, accumulations of paraffin, sand or scale may accumulate in the flow passages of the manifold body, thereby forming restrictions or blockages. Any interference in the flow passages adversely affects the accuracy of the pressure measuring instruments. Accordingly, it is a practice in the industry to remove accumulated scale by "rodding" the passageways in the manifold. Rodding refers to the procedure in which a long, thin metal rod is forced through the manifold flow passages to break free any accumulated solids, which are then flushed from the system.
As instrument manifolds have become more widely accepted, it has been suggested to incorporate two "block" valve assemblies and a "bleed" valve assembly into a single body to further reduce manufacturing and installation costs, and to eliminate possible leak points. The incorporation of a bleed valve assembly into the unitized body need not increase the bulk or weight of the manifold body, and creates an instrument manifold commonly referred to as a "double block and bleed" manifold. More particularly, it is conventional to install a small, hard seat "mini-valve" in the body above the main channel of the manifold body to provide a convenient method of venting the main channel to the atmosphere. A significant disadvantage of prior instrument manifolds having a unitized bleed valve assembly is that when some flow passages are rodded, the rodding process often drives and compresses small amounts of solids into other inaccessible flow passages of the manifold, resulting in disposal or disassembly of the manifold.
It is a further disadvantage of prior unitized manifolds that a "soft seat" valve assembly has typically been placed in a flow passage if that passage is to permit rodding. A soft seat valve assembly has a metallic conical plug which presses against the inside of an elastomeric seat fixed inside the valve chamber to effectively block the flow passages and seal against the passage of fluid through the valve. Although a soft seated valve assembly provides a good gas tight seal, it is severely limited by the pressure and temperature which the seat may tolerate. Typically, a soft seat valve may tolerate a maximum temperature of 200.degree. F. For some applications, a high temperature resin may be used to extend the temperature range to approximately 400.degree. F.; however, these higher temperature resins have inferior sealing capability. In some environments, such as steam measurement, the temperature and pressure in the manifold exceeds the tolerances of the elastomeric soft seat and the system fails.
Many instrument manifolds utilize a valve assembly with a conical plug, sometime having a ceramic ball tip, placed over a higher pressure vertical chamber in the manifold body to form a metal-to-metal seal. This metal-to-metal seal assembly, called a "hard seat" valve assembly, may tolerate working temperatures up to 1500.degree. F. A disadvantage of the conventional hard seat metal-to-metal valve assembly typically used in instrument manifolds is that it is not roddable. This type of manifold is not preferred by many operators because it permits the accumulation of solid material in the high pressure chamber of the valve assembly which is not accessible by a rod.